1. Field of the Invention
The present invention relates to an inverted-F antenna and a radio communication system equipped with the antenna and more particularly, to an inverted-F antenna capable of operation in separate frequency bands or a wide frequency band formed by overlapping separate frequency bands, and a radio communication system necessitating the switching of its operating frequency band, such as a digital portable or mobile telephone.
2. Description of the Prior Art
In general, mobile radio communication systems such as cellular phones exchange communications or messages by using one of assigned frequency bands.
In recent years, as the popularity of cellular phones has explosively grown, the exchange of communications or messages has become difficult to be performed by using a single specified frequency band. To cope with this situation, cellular phones tend to be equipped with a function enabling the communication/message exchange using separate frequency bands or a single wider frequency band.
Conventionally, an inverted-F antenna has been widely used as a receiving antenna of a cellular phone, because it can be formed compact. However, an inverted-F antenna has a disadvantage that the operable frequency band is comparatively narrow. Therefore, various techniques have been developed to make it possible for an inverted-F antenna to cover separate frequency bands or a wider frequency band.
For example, the Japanese Non-Examined Patent Publication No. 10-65437 published in March 1998 discloses an improvement of an inverted-F antenna, which was invented by the inventor of the present invention, T, Saito. This improved antenna is shown in FIGS. 1 to 3.
As shown in FIG. 1, the prior-art inverted-F antenna 110 is comprised of a rectangular conductor plate 100 serving as a radiating element, a circuit board 106 serving as a ground conductor, and a dielectric spacer 107 placed between the plate 100 and the board 106. The spacer 107 serves to fix the distance between the conductor plate 100 and the circuit board 106 at a specific value, thereby stabilizing the radiating characteristics of the antenna 110. The long-side length of the conductor plate 100 is La and the short-side length thereof is Lb.
The conductor plate or radiating element 100 has a feeding terminal 102 for feeding a Radio-Frequency (RF) electric signal to the element 100 or receiving a RF electric signal therefrom, a grounding terminal 103 for grounding he element 100 to the board or ground conductor 106, and a switching terminal 104 for switching the resonant frequency of the antenna 110. The radiating element 100 and the terminals 102, 103, and 104 are formed by a conductor plate. The terminals 102, 103, and 104 are L-shaped and connected to a short-side of the rectangular radiating element 100. The pitch between the terminals 102 and 103 is Lc. The pitch between the terminals 103 and 104 is Ld.
The lower part of the feeding terminal 102, which is bent to be parallel to the circuit board 106, is separated from the board 106 by a rectangular hole 106a penetrating the board 106. Therefore, the feeding terminal 102 is not electrically connected to the board 106. The lower part of the terminal 102 is electrically connected to a receiver circuit 108 in a radio section 120 of a cellular phone, as shown in FIG. 2.
The lower part of the grounding terminal 103, which is bent to be parallel to the circuit board 106, is contacted with and electrically connected to the board 106. The lower part is fixed to the board 106 by soldering. Thus, the terminal 103 is electrically connected to the ground.
The lower end of the switching terminal 104, which is bent to be parallel to the circuit board 106, is separated from the circuit board 106 by a rectangular hole 106b penetrating the board 106. The lower end of the terminal 104 is electrically connected to one terminal of a switch 105 located in the hole 106b. The other terminal of the switch 105 is electrically connected to the board 106.
The switch 105 is controlled by a controller circuit 109 in the radio section 120 of the cellular phone, as shown in FIG. 2. If the switch 105 is turned off, the switching terminal 104 is electrically disconnected from the circuit board 106, in which only the grounding terminal 103 is electrically connected to the board 106. If the switch 105 is turned on, the switching terminal 104 is electrically connected to the circuit board 106, in which not only the grounding terminal 103 but also the switching terminal 104 are electrically connected to the board 106.
When the switch 105 is in the OFF state, the perimeter L of the rectangular radiating element 100 is given as EQU L=(2La+2Lb).
In this case, as shown in FIG. 3, the VSWR (Voltage Standing-Wave Ratio) is minimized at a frequency f1. In other words, the resonant frequency of the antenna 110 is f1.
On the other hand, when the switch 105 is in the ON state, the equivalent electric length L' of the rectangular radiating element 100 is given as EQU L'.apprxeq.(2La+2Lb-Ld).
In this case, as shown in FIG. 3, the VSWR is minimized at a frequency f2 higher than f1. In other words, the resonant frequency of the antenna 110 is switched from f1 to f2.
Thus, the resonant frequency of the prior-art antenna 110 can be changed between f1 and f2 and accordingly, the cellular phone having the antenna 110 is capable of covering two separate frequency bands or a wide frequency band formed by overlapping the two separate frequency bands.
Although not shown here, the Japanese Non-Examined Patent Publication No. 62-188504 published in August 1987 discloses a patch antenna comprising two relatively-movable radiating elements in addition to a ground plate. An RF signal is fed to the ground plate by a coaxial feeding line. The two radiating elements can be overlapped and contacted with each other, thereby changing the total volume or dimension of the radiating elements. Thus, the resonant frequency of the prior-art patch antenna disclosed in the Japanese Non-Examined Patent Publication No. 62-188504 can be changed, thereby covering two separate frequency bands or a wide frequency band formed by overlapping the two separate frequency bands.
Recently, there arises a problem that the available frequencies assigned to cellular phones tend to be short due to the increased traffic. To solve this problem, a consideration that new frequency bands are assigned to cell phones in addition to the conventional assigned frequency bands has been made, thereby relaxing or decreasing the congestion.
To cope with this consideration, the above-described prior-art antennas have the following problems.
With the prior-art antenna disclosed in the Japanese Non-Examined Patent Publication No. 10-65437, the resonant frequency is changed by connecting or disconnecting electrically the switching terminal 104 to or from the circuit board 106. Therefore, to cope with a newly-assigned frequency band, another switching terminal needs to be provided to the radiating element 100. However, the addition of the switching terminal is not always possible.
For example, if a newly-assigned frequency band (e.g., 830 MHz-band or near) is located between the two conventionally-available frequency bands (e.g., 820 MHz- and 880 MHz-bands) and near one of these two frequency bands, a newly-added switching terminal needs to be provided between the grounding terminal 103 and the switching terminal 104 and at the same time, it needs to be located near one of the terminals 103 and 104. However, some specific limit exists in fabricating actually the prior-art antenna 110 with the detachable ground terminals. As a result, the prior-art antenna 110 is difficult to cope with the addition of a newly-assigned frequency band.
Also, in recent years, cellular phones have been becoming more compact and more lightweight. Addition of a new grounding terminal to the radiating element 100 enlarges the size of the antenna 110 and the cellular phone itself. Thus, it is difficult to ensure the distance or pitch between the newly-added grounding terminal and a nearer one of the grounding terminals 104 and 105.
Moreover, the newly-added ground terminal necessitates a new land for its electrical connection on the circuit board 106, which requires more labor. The formation itself of the new land is difficult, because patterned circuits have been closely arranged on the board 106.
With the prior-art patch antenna disclosed in the Japanese Non-Examined Patent Publication No. 62-188504, there is a problem that the volume of the antenna is unable to be utilize effectively because this antenna has two movable radiating elements.